CN0287 Datasheet, PDF (6/9 Pages) Analog Devices – Devices Connected

CN-0287
Circuit Note
Thermocouple Measurement Linearity
Figure 7 shows the approximate linearity of the type K
thermocouple system. The “cold junction” temperature is
0°C in this plot.
60
50
40
30
20
10
0
–10
–500
0
500
1000
TEMPERATURE (°C)
1500
Figure 7. Type K Thermocouple Temperature vs. Output Voltage with 0°C
Cold-Junction
The precision voltage for calibration as well as testing is
provided by the Fluke 5700A Calibrator high precision dc
voltage source with a resolution of 10 nV. The voltage error in
Figure 8 is within 0.2 µV of ideal, corresponding to about
0.004°C. This result is the short time accuracy result just after a
system calibration at 25°C without the effects of temperature
drift.The dominant error for this circuit is from the cold-
junction compensation measurement. In this circuit the
ADT7310 is used for cold-junction compensation and has a
typical error of −0.05°C, and a worst case error of ±0.8°C over
the −40°C to +105°C temperature range for a 5 V supply. The
device has a ±0.4°C maximum error over this temperature
range if a 3 V supply is used.
0.20
0.15
0.10
0.05
0
1 6 11 16 21 26 31 36 41 46 51
INPUT VOLTAGE (mV)
Figure 8. Error of CN-0287 Configured for Type K Thermocouple (VDD = 5 V,
VREF = 4.096 V, Differential Input, Bipolar, Input Buffer Enable, Output Data
Rate = 50 Hz, Gain = 32, Chop Enable. 60 Hz Rejection Enable, Sinc⁴)
RTD Configuration Test Results
For a Pt100 RTD, the default ADC gain setting is G = 8, and for
a Pt1000 RTD the default gain setting is G = 1. The reference
voltage to the ADC is equal to the voltage across the 4.02 kΩ
reference resistor. The temperature coefficient of a Pt100 RTD is
approximately 0.385 Ω/°C, and at +850°C the resistance can be
as high as 400 Ω. With a 400 µA default excitation current, the
maximum RTD voltage is therefore about 160 mV. The reference
voltage to the ADC is 4.02 kΩ × 400 µA = 1.608 V. For G = 8,
the maximum RTD voltage is 160 mV × 8 = 1.28 V which is
approximately 80% of the available range.
For a Pt1000 RTD, the maximum resistance at +850°C is
approximately 4000 Ω. The default excitation current is 380 µA,
yielding a maximum RTD voltage of 1.52 V. The reference voltage
to the ADC is 4.02 kΩ × 380 µA = 1.53 V. A default gain setting
of G = 1 is used, and the maximum RTD voltage utilizes nearly
all of the available range.
The general expression for the RTD resistance, R, in terms of
the ADC code (Code), resolution (N), reference resistor (RREF),
and gain (G) is given by:
R
=
Code
2^N

R _REF
G

(7)
The leakage current from TVS, diodes, clamping diodes, and
ADC are the largest sources of errors in the RTD measurement
circuit, even though nanoamp devices were selected for the design.
The total leakage current for each of the inputs is 9 nA (3 nA from
AD7193, buffer on), 5 nA from clamping diode and 1 nA from
the TVS diode). All four channels will thus generate 36 nA
maximum leakage current. The feedback loop in Figure 2 maintains
a constant current through the reference resistor. This means
that leakage currents affect the RTD excitation current, thereby
producing an error. The default exciting current is 400 µA for
Pt100 and 380 µA for Pt1000. The approximate worst case system
error due to the leakage currents for Pt100 RTDs is:
Error(%)
=



36 nA
400 μA


×100

≈
0.01%
of
reading
(8)
For a Pt100 with measurable range from −200°C to +850°C, this
corresponds to a system accuracy of approximately
Accuracy ( C ^ ) = ^{400 Ω} × 0.0001 ≈ 0.1C ^
(9)
0.385 Ω / C ^
The amount of the error depends on the configuration of the
input terminals. After an input configuration is established, a
room temperature calibration can reduce the error even further.
An experiment was conducted to show the effects of leakage
current. Each channel was first configured as a 4-W RTD. A
100 Ω fixed resistor was connected to Channel 1 in the RTD
position. Zero ohm resistors were connected to the inputs of the
other three channels.
Rev. C | Page 6 of 9